1. Design and Fabrication of Power Scissor Jack

2. Introduction  Jack : Jack is a mechanical device used to lift heavy loads or apply great forces. A mechanical jack employ a square thread for lifting heavy equipment.  The most common form is a car jack, floor jack or garage jack which lifts vehicles so that maintenance can be performed. Mechanical jacks are usually rated for a maximum lifting capacity (for example, 1.5 tons to 3 tons).  The car jack in our project is generally termed as Scissor Jack because of its shape which is much like a scissors.

3. Objectives  To design a power scissor jack which is safe and reliable to raise and lower the load easily.  To fabricate the prototype of a scissor jack which is operated by a gun powered by the car battery.

4. Scissor Jack  A scissor jack is a device constructed with a cross-hatch mechanism, much like a scissor, to lift up a vehicle for repair.  The main parts of scissor jack are top plate, bottom plate, arms, trunions and power screw  Most scissor jacks are similar in design, consisting of four links driven by a power screw.  The power screw design of a common scissor jack reduces the amount of force required by the user to drive the mechanism.

5. Power Screws  Mechanical device used for converting rotary motion into linear motion and transmitting power.  Three essential parts of a power screw are screw, nut and a part to hold either the screw or the nut in its place. Applications  To raise the load, e.g. screw-jack, scissor jack,  To obtain accurate motion in machining operations, e.g. lead-screw of lathe,  To clamp a work piece, e.g. vice, and  To load a specimen, e.g. universal testing machine.

6. Advantages  Mechanical Advantage  Large Load Carrying Capacity  Compact construction  Simple to design  Fabrication is easily done on a lathe machine  It can be designed with a self locking property Disadvantages  Power screws have very poor efficiency; as low as 40%  High Friction causes rapid wear of thread

7. Forms of Threads Square Thread Trapezoidal Thread Acme Thread

8. Advantages  Efficiency of square threads is more than that of trapezoidal threads  Due to the lack of a thread angle there is no radial pressure on the nut Disadvantages  Square threads are difficult to manufacture  Square threads have less thickness at core diameter, which reduces its strength Square Thread used in high load applications such as lead screws and jackscrews. It gets its name from the square cross-section of the thread. It has the lowest friction and is most efficient thread form.

9. Terminology of Power Screw Pitch: ‘p’, The distance measured parallel to the axis of the screw, from a point on one thread to the corresponding point on the adjacent thread. Lead: ‘L’, The distance measured parallel to the axis of the screw that the nut will advance in one revolution of the screw. For a single-threaded screw, the lead is same as the pitch, for a double-threaded screw, the lead is twice that of the pitch. Nominal diameter: ‘d o ’ the largest diameter of the screw. It is also called major diameter. It is denoted by the letter. Core diameter: ‘d c ’ The smallest diameter of the screw thread. It is also called minor diameter. Helix angle: ‘α’ The angle made by the helix of the thread with a plane perpendicular to the axis of the screw. Helix angle is related to the lead and the mean diameter of the screw. It is also called lead angle.

10. Self Locking Screw  When   , a positive torque is required to lower the load  Under this condition, the load will not turn the screw and will not descend on its own unless effort P is applied. This is called self locking.  “A screw will be self-locking if the coefficient of friction is equal to or greater than the tangent of the helix angle”  self-locking of screw is not possible when the coefficient of friction (μ) is low.

11. Efficiency  Efficiency η = Work output/ Work input = W x L/ P x (π d) = (W/P)* tan (  ) = tan (  )/tan (  +  )  For our scissor jack prototype η = tan (4.85)/tan (11.3+4.85) = 0.28 = 28% There are two ways to increase the efficiency of square threaded screws. They are as follows: 1.Reduce the coefficient of friction between the screw and the nut by proper lubrication 2.Increase the helix angle up to 40 to 45° by using multiple start threads.

12. DESIGN W

13. Power Screw  Material selected Mild Steel  Assumptions The weight of the car is considered as 1.5 ton. The weight acting on front and rear axle is 60% and 40% of total weight respectively, hence the weight acting on front axle i.e.; 900 kg is considered for designing the jack. A weight of 450 kg acts on each wheel. And the maximum load on screw act when jack is at its lowest position. A double start square thread with a friction coefficient of 0.20 is considered.

14. Design Calculations Length of each arm = L1 =L2 =L3 =L4 =160mm Length of the power screw = (w1+w2+w3) = 350mm w1 = w3 = 150 mm w2 = 50 mm Maximum lift of the jack = (h1+h2) = 300 mm  is the angle made by link with horizontal when jack is at its lowest position. cos (  ) = (175-25)/160 = 20.36˚ W = (load * g) = (450*10) = 4500 N = 4.5 kN The tension T acting on the power screw is shown in the Fig. Tension, T = W/2*tan (  )

15. Total tension = 2*T = W/tan (  ) For a power screw under tension we can take  t = 124 N/mm 2 for mild steel Let d c be the core diameter of the screw. But load on the screw is Load = (π/4)* d c 2 *  t So, 2*T = W/tan (  ) = (π/4)* d c 2 *  t 2*T = 4.5 kN/tan (20.36˚) = 12123.44 N d c 2 = (W/tan (  ))*(4/ (π*  t )) Hence, d c = 11.34 mm Since the screw is subjected to torsional shear stress we adopt, d c = 14 mm Taking pitch, P = 2 mm Outer diameter, d o = d c + P = (14+2) = 16 mm Mean diameter, d = d o – P/2 = 16-2/2 = 15 mm.

16. Check for self-locking tan (  ) = Lead/π*d;  = helix angle Lead L = 2*P; since the screw has a double start square thread. tan (  ) = 2*p/π*d = 2*2/ π*15 = 0.084 Helix angle;  = 4.85˚ Coefficient of friction; μ = tan (  ) = 0.20; friction angle;  = 11.3˚  >  hence the screw is self-locking Effort required to support the load = 2*T tan (  +  ) = 12123.44 (tan (  ) + tan (  ))/ (1- (tan (  ) * tan (  ))) = 3510.715 N Torque required to rotate the screw = effort *d/2 = 3510.715 * 15/2 = 26330.36 N-mm

17. Shear stress in the screw due to torque  = 16*T/ (π* d c 3 ) = 16*26330.36/ (π*14 3 ) = 48.87N/mm 2 But tensile stress  t = 2*T/ (π/4) * d c 2 = 12123.44/ (π/4) * 14 2 = 78.755 N Maximum principal stress  t max =  t/2 +  (  t 2 +  2 )/2 = 102.13 N/mm 2 Maximum shear stress  max =  (  t 2 +  2 )/2 = 62.76 N/mm 2 Since the maximum stresses  t max and  max within the safe limits, the design of double start square threaded screw is satisfactory

18. Nut Material Selected Bronze Design Calculations Let n be the number of threads in contact with the screw assumed that load is Uniformly Distributed over the cross section area of the nut. Bearing pressure is assumed as 15 N/mm 2 P b = (2*T)/((π/4)*(do 2 -dc 2 )*n) 15 = (12123.44)/ ((π/4)*(16 2 -14 2 )*n) Number of threads, n = 10.6 ≈ 11 Thickness of Nut = n*p = 11*2=22 mm Width of Nut b =1.5*do =1.5*16=24 mm To control the movement of nuts beyond 300 mm the rings of 8 mm thickness are fitted on the screw with the help of set screw The length of screw portion = 300 + (8*2) + 22 = 338 mm ≈ 350 mm Total length of screw is 350 mm.

19. Pins in Nut Material selected Mild Steel Design calculations Let d 1 = diameter of pins in the nuts Since Pins are in double shear stress Load on pins = W/2 = 2*(π/4)*d 1 2 *  =12123.44/2  = Shear stress = 50 MPa for steel Hence d 1 = 8.78 mm ≈ say 10 mm Diameter of pins head is taken as 1.5*d 1 = 15 mm and thickness be 4 mm

20. Arms Material selected Mild Steel Design calculations σ yt for mild steel = 248 N/mm 2 Factor of safety (F.S) = 2.5 σ t = σ yt /F.S=248/2.5=99.2 N/mm 2 σ c = 1.25*σ t = 1.25*99.2 = 124 N/mm 2 Cross section area (A) = (40*3) + (24*3) + (40*3) = 312 mm 2 Moment of Inertia I xx = 47376 mm 4, I yy = 51009.38 mm 4 Radius of Gyration R x = 12.323 mm, R y = 12.786 mm Rankine’s constant (a) =1/7500 Ends are hinged (L eff = L) P­ cr in vertical plane σ c = crippling stress = 330 N/mm 2 P­ cr = (σ c *A)/(1+a*(L/ R y ) 2 )= (330*312)/(1+(1/7500)*(160/12.786) 2 = 100854.26 N P­ cr in horizontal plane σ c = crippling stress = 330 N/mm 2 P­ cr = (σ c *A)/(1+a*(L/2*R x ) 2 )= (330*160*40)/(1+(1/7500)*(160/2*12.323) 2 )= 2100198.258 N Since Buckling load is more than Design load the dimensions of the link safe.

21. Top Plate (Loading Platform) Material used Mild Steel Design calculations Moment, M = (p*l)/4 p = 5000 N l = 50 mm M = (5000*50)/4 = 250000/4 = 62500 N-mm Z = (b*h 2 )/6 = (36*40 2 )/6 = 9600 mm 3 b = 36 mm, h = 40 mm σ b = M/Z = 62500/9600 = 6.51 N/mm 2 Conclusion The permissible stress for mild steel is 124 N/mm 2 and it is greater than σ b = 6.51 N/mm 2.The top plate design is safe.

22. Bottom Plate (Support) Material used Mild Steel The size and shape of the bottom plate have been selected to provide the stability to the power Scissor Jack. The size of bottom plate has been fixed as 120*70*3 mm.

23. Drawings Power Screw

24. Trunion

25. Top Arm

26. Bottom Arm

27. Top Plate

28. Bottom Plate

29. Assembly Drawing

30. Exploded Front View

31. Exploded Side View

32. Bill of Materials

33. Manufacturing Methods  Production of Screw Threads – Possible Methods  Casting  Forming (Rolling)  Removal process (Machining)  Semi finishing and finishing (Grinding)  Precision forming to near – net – shape  Non-conventional process (EDM, ECM etc.)

34. Processes, Machines and Tools Used For Producing Screw Threads  Machining  Rolling  Grinding

35. FABRICATION Top Arms and Bottom Arms S no.MachineOperationTools Time (min) 1 Stores Check the raw material Try square, steel rule, and dot punch 20 2 Welding shop Welding of a flat plate to the angular to obtain channel section. Welding gun, Files and Emery paper 120 3 Grinding machine Grinding the plate in vice Grinding wheel60 4 Radial Drilling machine Drilling 10 mm holes at both the ends of the plate Drill bit, dot punch, hammer and steel rule 40

36. power screw S no.MachineOperationTools Time (min) 1 StoresCheck the raw material Outer calipers, steel rule 5 2 Sawing machine Cutting the length of the rod as per requirement Hack saw25 3 Lathe machine Turning the diameter to 16 mm Single point cutting tool 35 4 Lathe machine Threading of square thread Threading tool60 5 Shop FloorInspectionVernier calipers5

37. Trunions S no. MachineOperationTools Time (min) 1 StoresCheck the raw material Inner calipers, steel rule 5 2 Sawing machine Cutting the length of the rod as per requirement Hack saw25 3 Lathe machine Turning the outer diameter to 24 mm Single point cutting tool 35 4 Lathe machine Boring the Trunions to 16mm diameter Boring tool15 5 Lathe machine Threading of square thread Internal Threading tool 60 6 Shop FloorInspectionVernier calipers5

38. Top and Bottom Plates S no. MachineOperationTools Time (min ) 1 StoresCheck the raw material Try square, steel rule, dot punch 15 2 Welding shop Welding of a flat plate to the angular to obtain channel section. Welding gun, Files and Emery paper 120 3 Grinding machine Grinding the plate in vice Grinding wheel90 4 Radial Drilling machine Drilling 10 mm holes at both the ends of the plate Drill bit, dot punch, hammer and steel rule 60 5 Shop FloorInspectionVernier calipers 10

39. Conclusion and Scope for future work Conclusion In this project a prototype of power scissor jack which can be operated by a power gun has been designed and fabricated. The jack has been designed to a pay load of 4.5kN. The salient features of the present fabrication are elimination of human effort to operate the jack, through a simple electrical device which can be actuated by a 12 V battery and provision of a light source to facilitate convenient operation during night time. All the elements of the jack are fabricated in the machine shop. The assembly of the component can be achieved in 100 minutes. Another feature of the unit is provision of two trunions on both the sides of the jack to ensure jerk free operation. The elements which are useful are readily available commercially for each and early replacement of failed components if required. Scope for future work As a development the web part of the arms can be replaced by stiffening ribs to reduce the overall weight.the top and base plates can be made foldable to make the unit more compact. Permanently mounted jacks on the vehicle can be developed so that tire change can be completely automated.